1. Field of the Invention
The present invention relates to a screw expander.
2. Description of the Related Art
Power generation systems in which a generator is driven by the flash of steam are widely introduced. Conventionally, there have been many large-scale facilities using turbo and axial-flow turbines. However, in terms of saving energy, there have been recently increasing needs for small-scale power generation systems for generating power by recovering exhaust heat.
The use of a screw expander instead of a turbine is known to be efficient in small-scale facilities as disclosed, for example, in “Study on Fundamental Performance of Helical Screw Expander” in Transactions of the Japan Society of Mechanical Engineers (Series B), pp. 134-142, No. 461, Vol. 51, published on Jan. 1985. Generally, in a screw expander, a ratio of a volume during intake to a volume during exhaust is determined by a mechanical shape and an internal expansion ratio as a ratio of an intake pressure and an exhaust pressure inside is constant. Thus, as disclosed in the above literature, a loss occurs if the internal expansion ratio of the screw expander does not coincide with an operation expansion ratio as a ratio of a pressure at an intake side and a pressure at an exhaust side.
As a means for adjusting an internal expansion ratio of a screw expander, there is a method for changing an exhaust position by a slide valve as disclosed in Japanese Unexamined Patent Publication No. S62-60902. However, this method is disadvantageous in that a mechanism for driving the slide valve is required and an apparatus becomes complicated and large.
As a system for generating power by low-temperature heat with which flash power generation cannot be utilized, there is a binary power generation system in which a turbine and an expander are driven by a low-boiling heat medium, for example, as disclosed in U.S. Pat. No. 4,608,829. Since the binary power generation system has, in principle, low power generation efficiency, it is hardly put to practical use except for such cases where a huge heat source is present despite having such a temperature at which steam cannot be flashed as in geothermal power generation.
However, if a small-size binary power generation system can be inexpensively provided, heat which has not been conventionally utilized at all, e.g. heat discarded to cool a cylinder block of an internal combustion engine can also be recovered as electrical energy. To make such a power generation system economically rational, it is very important to make the screw expander more efficient.
Japanese Patent No. 3904852 discloses a screw compressor which has a simple structure and can reduce a starting torque and smoothly start without causing overload of a motor by providing a piston valve which allows a space at an intake side and an intermediate pressure portion to communicate using an intake pressure and an exhaust pressure as drive forces. This can be said to disclose a screw expander whose mechanical compression ratio (internal compression ratio) changes only at the time of start, but does not disclose a technology applicable to a screw expander as it is.